1. Field of Invention
This invention relates to the area of EEG (Electroencephalography) and the study of evoked brain potentials.
2. Description of Prior Art
A system to define a standard set of electrode placements on the scalp, so that results obtained in different clinics and laboratories could be compared, was devised by Herbert H. Jasper, M.D. and subsequently adopted for trial at the meeting in 1949 of the General Assembly of the International Federation in Paris. Dr. Jasper's scheme defines a set of electrode placements on the scalp whose relative position will be determined by the dimensions of each individual's head so that electrodes placed on heads of different dimensions will be in comparable locations on the scalp (Jasper, 1958). This system is based on a set of latitudinal and longitudinal arcs upon the surface of an approximately spherical cranium, and positions of electrodes are determined by measurements from standard landmarks (i.e., nasion, inion, and pre auricular points) on the skull. Arc length measurements from nasion to inion (FIG. 3) and from the pre-auricular point of one ear over vertex to the opposite ear (FIG. 4) are taken as well as a measurement around the circumference (FIG. 5). Electrodes are then placed at locations 10, 20, 20, 20, 20, and 10% along each of these arcs as shown in FIGS. 3, 4, and 5 respectively. Thus, the name of this system has been designated as "the 10/20 international system".
Heretofore, the placement of electrodes on the scalp according to the International 10-20 system has been ascertained by mainly one of two methods: (1) the laboratory technician measures each arc length with a flexible tape (such as one used by a seamstress or tailor), then calculates the appropriate 10 and 20 percentages (often the 20% interval is set using a pair of blunt tipped calipers) to locate the appropriate sites for electrodes that are subsequently affixed on the scalp; (2) Electrode caps are employed to record the electrical activity from the scalp.
An electrode cap (U.S. Pat. No. 4,025,739) was described by Mr. Marvin Sams April 1978, and subsequently another by Mr. Gary W. Sherwin (U.S. Pat. No. 4,709,702) Dec. 1985. Presently electrode caps of various sizes, according to the 10/20 system, have been designed and are marketed by Electro-Cap International, Inc. in Eaton, Ohio (FIG. 1) and perhaps others; however, a simple tool, such as the "electrode locator" that will complement basic measurement procedures and reduce electrode application time has not been devised and introduced to the scientific community. Electrode cap systems accommodate various head sizes, but to do so can require as many as seven different caps; and still there remains head circumferences outside the range of standard caps. The cost of this system is an important factor to be considered. In many instances, especially in research studies, a limited number of specified electrode sites are assessed and a cap is not a consideration. The cap cannot be used in clinical applications where skull defects are present, in situations where pressure on the skull is detrimental, or on severely injured patients that cannot be fixed with straps (Blom and Anneveldt, 1982). Many users, therefore, would find it desirable to have a simple tool that could facilitate the electrode placement on the scalp at relatively low cost.